1. Field
The present teachings relate to amplifiers. More particularly, the present teachings relate to a variable impedance match and variable harmonic terminations for different modes and frequency bands.
2. Description of Related Art
A transistor, such as a field effect transistor (FET), that is configured to operate as a small signal amplifier and, for example, accepts a pure sine wave (e.g. no high order harmonic components) at an input (e.g. gate) terminal and produces a nearly pure sine wave at an output (e.g. drain) terminal of the transistor and will produce minimal harmonic components at the output terminal of the transistor. However, a transistor that is configured to operate as a switching power amplifier that produces a non-sinusoidal waveform at an output terminal of the transistor will produce harmonic components at the output terminal. Non-sinusoidal waveforms include such waveforms as a clipped sine wave, a square wave or near square wave, among other possibilities. As is known by a person skilled in the art, certain harmonic components can degrade linearity and efficiency characteristics of the transistor that is configured to operate as an amplifier.
As used in the present disclosure, the term “couple” can refer to a degree by which various electronic components affect each other, without being necessarily physically connected. For example, a capacitor connected in series with a resistor at the output (terminal) of an amplifier, the resistor being connected to the amplifier and the capacitor being connected to ground, the capacitor will affect an output signal generated by the amplifier. This capacitor is coupled to the output of the amplifier. In some cases, coupling can be used within the context of an entire circuital arrangement, for example an amplifier fitted with an output filter containing various components. If all the components of the filter provide a contribution to the filter response, thus affect the output signal of the amplifier to provide a modified signal at the output of the arrangement, then as used in the present disclosure, all the filter components are coupled to the amplifier output.
As used in the present disclosure, the term “decouple” can refer to a lack of any affect or influence between various electronic components, even if some of these components are physically connected. For example, a capacitor connected to an output (terminal) of an amplifier, with one side directly connected to the amplifier output and the other side connected to ground, is coupled to the amplifier and will affect an output signal generated by the amplifier. Removing the ground connection of the capacitor will remove the electrical influence of the capacitor over the output signal as the latter will become completely independent of the capacitor, although still physically connected to it. In this case, the capacitor is decoupled from the amplifier (output). Using the example above of the circuital arrangement, a filter component that does not affect the response of the filter, and thus does not affect the output signal of the amplifier, as used in the present disclosure is said to be decoupled from the amplifier output.
The output terminal of an amplifier device can be coupled to one or more waveform shaping elements (e.g. filters) that for example attenuate the even harmonics and enhance the odd harmonics, for improved linearity and efficiency. Throughout the present disclosure such waveform shaping elements will be referred to as harmonic terminations, or harmonic impedance terminations, interchangeably. Harmonic shorts and harmonic opens are examples of harmonic termination. An even harmonic short at the drain output terminal can shunt an even harmonic component of an input signal present at the drain output terminal to ground, effectively removing or attenuating the even harmonic component from the input signal. An odd harmonic open at the drain output terminal can prevent the amplifier from seeing any load other than the output impedance of the amplifier itself at the corresponding odd harmonic frequency. For discussion purposes, a waveform of the voltage signal at the drain output terminal may be referred to as a drain voltage waveform whereas a waveform of the current signal at the drain output terminal may be referred to as a drain current waveform. Either waveform may be subject to waveform shaping.
Odd order intermodulation can impact the linearity characteristics of a system. Attenuation of the second harmonic (the first even harmonic waveform shaping element), via for example a second harmonic short, can improve linearity since the second harmonic can mix with the fundamental to generate third-order intermodulation products close to the operating frequency of the amplifier and thus affect linearity. A fourth harmonic short can improve linearity by attenuating the fourth harmonic, which can generate fifth-order intermodulation products and thus affect linearity. Similarly, an nth harmonic short, where n is an even integer, can improve linearity by attenuating the nth harmonic, which can generate (n+1)th-order intermodulation products and thus affect linearity.
Generally, the fifth-order intermodulation products are considerably smaller in magnitude than the third-order intermodulation products and similarly, the seventh-order intermodulation products are considerably smaller in magnitude than the fifth-order intermodulation products. Therefore, eliminating third-order intermodulation is generally more effective, and in many cases sufficient, to maintain linearity.
Second-order intermodulation products generally are outside a passband of the amplifier and therefore can generally be ignored. Fourth-order and higher even order intermodulation products are generally small in magnitude and outside a passband of the amplifier and therefore can generally be ignored.
The combination of even harmonic shorts and odd harmonic opens can shape the drain voltage waveform and the drain current waveform to provide higher efficiency by reducing overlap of drain voltage and drain current waveforms. A square wave with 50% duty cycle has no even harmonic components, and thus a combination of second and fourth harmonic shorts together with third and fifth harmonic opens provides a near square wave drain voltage waveform.
Generally, an amplifier output match can be connected between an output terminal of the amplifier and a load in order to provide impedance matching between the output terminal of the amplifier and the load. The amplifier output match can be different for different modulation and coding schemes (e.g. GMSK (Gaussian minimum shift keying), 8PSK (eight-phase shift keying), H-PSK (hybrid phase shift keying), orthogonal frequency multiplexing and DFT-spread orthogonal frequency multiplexing), such as are utilized in cellular access standards (e.g. GSM, EDGE, WCDMA, LTE, and so forth) because power level and linearity requirements are different for each modulation and coding scheme.
Many amplifiers, such as power amplifiers, employed in today's cellular phones have a fixed-tuned output match (e.g. constructed using components of fixed values), which restricts operation to a single frequency band and a single modulation and coding scheme. As known by one skilled in the art, the term “frequency band” may refer to a range of frequencies that is around the operating frequency and that extends above and below the operating frequency by certain deviation(s) and the term “modulation and coding scheme” may refer to a particular method of modulating a signal to encode information. In many applications, it may be desirable to have amplifiers capable of operating on more than one frequency band and/or modulation and coding scheme. Switching and/or tuning the harmonic terminations (e.g. harmonic opens and harmonic shorts) and/or the amplifier output match based on frequency band and/or modulation and coding scheme can provide optimum results for a given frequency band and/or modulation and coding scheme.